A pseudo-wire is a mechanism that emulates the essential attributes of a telecommunications service (such as a T1 leased line or Frame Relay) over a variety of Packet Switched Network (PSN) types. A pseudo-wire is intended to provide only the minimum necessary functionality to emulate the service with the required degree of faithfulness for the given service definition. The required functions of pseudo-wires include encapsulating service-specific bit streams, cells, or protocol data units (PDUs) arriving at an ingress port of a networking device and forwarding them across an Internet Protocol (IP) path or Multi Protocol Label Switching MPLS tunnel. In some cases, it is necessary to perform other operations within the pseudo-wire, such as managing the timing and order of data transmissions and to emulate the behavior and characteristics of the service to a required degree of faithfulness.
From the perspective of Customer Equipment (CE) devices, a pseudo-wire is characterized as an unshared link or circuit of the chosen service. In some cases, there are deficiencies in the pseudo-wire emulation that impact the traffic carried over a pseudo-wire. These deficiencies limit the applicability of this technology. These limitations must be fully described in the appropriate service-specific documentation and can make pseudo-wires unsuitable for some service or customers.
One service that can be impacted over a pseudo-wire is operations, administration and management (OAM) functionality. Some OAM functionality requires that OAM data be handled in the same manner as other data over the pseudo-wire. Virtual Circuit Connectivity Verification (VCCV) is an OAM function that is impacted by multi-path decision making in the PSN. Current pseudo-wires have limited or restricted support for VCCV, because the OAM messages such as OAM alert label VCCV (type 2) and time-to-live expiry VCCV (type 3) are not in-band, thus they are not ensured to be routed the same way as other traffic across the PSN in the pseudo-wire. VCCV using a control word (type 1) is in-band, but limits multi-path routing to the use of information of the pseudo-wire label stack.